1. Field of the Invention
The present invention relates to a process for producing an epoxidized diene polymer which is used for coatings, resin modifiers, rubber modifiers, adhesives, and so on. More particularly, the present invention relates to a process for producing an epoxidized diene polymer, in which when performing an epoxidation of a double bond in the diene polymer chain, the diene polymer is dispersed and suspended in an insoluble medium such as water or the like and is epoxidized in the presence of powder particles insoluble in the medium or in the presence of both the powder particles and a thermo-stabilizer, to thereby obtain the epoxidized diene polymers with a low gel content and/or with an excellent thermal stability.
2. Description of the Related Art
Heretofore, as a process for producing an epoxidized diene polymer by the oxidization of a diene polymer to be epoxidized, the following methods have been known in the art:
(i) a method in which a percarboxylic acid is previously prepared as an epoxidation agent by reacting hydrogen peroxide with a lower carboxylic acid such as formic acid or acetic acid, and the percarboxylic acid is then added in a reaction system to initiate an epoxidation reaction in the presence or absence of a solvent; and
(ii) a method in which an epoxidation is performed using hydrogen peroxide in the presence of a catalyst such as osmium salt or tungstic acid and a solvent.
Each of the methods (i) and (ii) is characterized by dissolving a diene polymer to be epoxidized in a solvent to effectively execute an epoxidation reaction. However, there is a disadvantage in that the recovery of products is extremely troublesome because of complicated steps of dissolving the diene polymer to be epoxidized in the solvent, and performing a washing treatment with water to eliminate by-products such as carboxylic acids and desolvation operation. In particular, when the organic polymer to be epoxidized is a rubber polymer, there is a problem in that the epoxidized products have adhesive properties. That is, the adhesive properties of such products will cause blocking of the products just after production, difficulty in handling, workability being changed for the worse, blocking at the time of storing the products, and so on.
Furthermore, depending on the product form, a usage mode is limited. When the product is used as a modifying agent, it may become a bale form in which the product cannot be manufactured as powder, clam, pellet, or the like after denaturing by epoxidation.
Regarding the process for producing an epoxidized diene polymer, the present inventors and coworkers have provided many proposals. In JP-A-08-120022, for example, there is proposed a method comprising the following steps: (1) mixing a diene polymer and/or partially hydrogenated product thereof with an organic solvent to obtain an organic solvent slurry or an organic solution of the diene polymer; (2) epoxidizing a double bond in the diene polymer using an epoxidation agent; (3) neutralizing and/or washing the epoxidation-reaction solution; (4) stripping a solution with 5 to 50 wt % of the epoxidized block copolymer in the presence of a surfactant at a temperature equal to or higher than the boiling point of the organic solvent, or at an azeotropic temperature or higher if the organic solvent and water form an azeotropic mixture, but not higher than 120° C. to obtain a slurry in which the epoxidized block copolymer is dispersed in water; (5) dehydrating the clam of epoxidized block copolymer containing water obtained in the previous step to a water content of 1 to 30 wt %; and (6) drying the epoxidized block copolymer obtained in the previous step to a water content of 1 wt % or less.
In JP-A-09-60479, there is proposed a method in which a screw-extruder type throttle dehydrator is used in the above drying step (6). In JP-A-09-95512, there is proposed a method for recovering epoxidized block copolymer, in which an organic solvent is removed through a direct vaporization by supplying the epoxidized block copolymer obtained in the above step (3) into a vaporizer. Further, in JP-08-104709, there is proposed a process for producing an epoxidized block copolymer having an improved gel content by specifying an acid number of a product.
However, each of these methods of the inventions relates to the epoxydized block copolymer in accordance with a uniform solution method for the epoxidation after dissolving the polymer (raw material) in the solvent, which is characterized by producing the product having a low gel content. The handling of high viscosity solution is full of difficulty. Furthermore, epoxidized block copolymers obtained by those methods have comparatively low softening points, respectively. Therefore, during the production, processing, transportation, or usage of such polymers, any trouble maybe caused in handling. For example, the pellets of the copolymer become blocked on their surfaces with each other or strongly adhered with each other.
As a method to cope with the problems, JP-A-09-67502 proposes one characterized by the addition of a blocking-preventing agent to the obtained epoxidized copolymer.
In JP-A-09-208617, there is proposed a chemically modified diene polymer composition and a preparation method therefor, where the polymer in the form of micro particles having a size of 0.05 to 10 ìm in water is denatured with epoxidation. However, it only discloses the process for producing the composition, so that there is no description about the recovery of epoxidized polymer, the improvement on blocking of the product, or the like. In JP-A-10-298232, furthermore, there is proposed a method for the epoxidation in an aqueous dispersion. However, the method is provided for specifically epoxidizing only the surface of polymer particle. Both applications do not mention about the heat stability of the obtained epoxy chemical product alone.